Communication systems, such as CATV systems or local area networks (LAN's) often require the use of line extenders and distribution amplifiers, such as bridging amplifiers in trunk repeater stations. Such devices must amplify incoming signals with little or no distortion and deliver the resulting signals to one or more distribution cables at medium to high radio frequency output levels. Further, this goal must be achieved at the lowest possible cost. Prior attempts at designing broadband amplifiers meeting these requirements have utilized feed-forward techniques. For example, Wagner, U.S. Pat. Nos. 4,677,390 and 4,812,779 disclose a feed-forward amplifier wherein a sample of an input signal supplied to a main amplifier is compared with a sample of an output signal developed by the amplifier to produce an error signal proportional to the difference therebetween. The error signal is amplified and combined in opposite phase with the main amplifier signal to substantially cancel distortion and noise products generated by the main amplifier.
Other types of feed-forward amplifiers are disclosed in O'Neil, et al., U.S. Pat. No. 3,886,470, Preschutti, U.S. Pat. No. 4,517,521 and Hsu, U.S. Pat. No. 4,782,307.
Blauvelt, et al., U.S. Pat. No. 4,992,754 discloses an electronic circuit that provides a linear output from an amplitude modulated transmission device, such as a solid state laser having inherent distortion. The distortion of the laser is compensated by applying a predistorted signal equal in magnitude and opposite in sign to the distortion introduced by the laser. An input signal is split into first and second paths and a primary part of the input signal is applied via the first path to the laser with a time delay to compensate for delays in the second path. A predistorter in the second path generates predistortion at an amplitude that matches the amplitude of the distortion. A phase adjust circuit is provided in the second path to allow precise control of the phase of the predistorted signal. This patent also discloses that other nonlinear devices, such as amplifiers, may have inherent distortion largely canceled by the technique disclosed therein.
It has been found that conventional feed-forward circuits are useful to provide amplification with low distortion. However, such circuits have relatively complex designs. Also, when it is desired to distribute the input signal to multiple utilization devices, multiple feed-forward circuits must be used, in turn undesirably adding to the complexity and cost of the overall system.